Heavy and extra heavy oil and bitumen represent the largest deposit types of recoverable hydrocarbons in the world. As an example, the proven, recoverable heavy oil reserves (including oil sands) in Alberta, Canada are greater that all of the light oil reserves of the Middle East. As used herein, heavy and extra heavy oil refers to a hydrocarbon-containing material having an American Petroleum Institute (“API”) gravity, or specific gravity, of no more than about 22.5° API, and bitumen to a hydrocarbon-containing material having an API gravity of no more than about 10° API. By way of comparison, light crude oil is defined as having an API gravity higher than about 31.1° API, and medium oil as having an API gravity between about 22.3° API and 31.1° API. Bitumen will not flow at normal temperatures, or without dilution, and is “upgraded” normally to an API gravity of 31° API to 33° API. The upgraded oil is known as synthetic oil.
To recover heavy oil and bitumen, its viscosity is reduced. In one common commercial method of recovering heavy oil and bitumen, steam is injected under pressure into the oil-bearing formation. The steam heats up the formation, including the oil and/or bitumen, causing it to flow under the force of the steam (and other fluid(s)) pressure to a recovery well where it is pumped to the surface for refining. In one steam-assisted technique, known as SAGD, or Steam Assisted Gravity Drainage, steam is used to heat the oil which then flows downward (under the force of fluid pressure and gravity) to horizontal recovery wells placed beneath the oil formation. Another heavy oil recovery method ignites injected gas to create a high temperature, high pressure firefront which sweeps through the oil formation, pushing some of the oil ahead of it. In other heavy oil recovery methods, various forms of fluid injection (such as carbon dioxide, water, steam, surfactants (which reduce the viscosity of the fluid layer between the oil and the ground formation), alkaline chemicals, polymers, etc.) are performed.
The use of electromagnetic energy (usually electrical or Radio Frequency or RF) to heat the heavy oil formation has been known for several years. This technology was introduced during the 1970s when there was widespread interest in exploiting oil shale reserves. There have been several variations of this technology, ranging from relatively low frequency through radio frequency and microwaves. These have included multi-probe “closed” field heating arrangements, single probe heating arrangements, and radiating configurations.
By way of example, U.S. Pat. No. 2,799,641 to Bell discloses a method for production enhancement through electrolytic means whereby a direct electrical current causes oil flow through electro-osmosis. Another electro-osmosis technique is disclosed in U.S. Pat. No. 4,466,484 to Kermabon. Other disclosures (for example U.S. Pat. No. 3,507,330 to Gill, U.S. Pat. No. 3,874,450 to Kern, and U.S. Pat. No. 4,084,638 to Whitting) describe attempts to heat the near-wellbore region as well as more distant parts of the reservoir by electrical methods.
Kasevich in U.S. Pat. No. 4,301,865 disclosed the use of an underground array of RF emitting rods, which enclose a defined volume that is to be heated. The array is used specifically for the recovery of oil shale kerogen.
Bridges, et al., in U.S. Pat. Nos. 4,140,180; 4,144,935; 4,790,375; 5,293,936; 5,621,844; 4,485,868; and 5,713,415, disclose arrangements of underground RF heating elements and associated transformer and cable equipment, all applicable to volumetric heating of a closely defined space at or near the production well.
Elligsen, in U.S. Pat. No. 6,499,536, suggests the injection of RF absorbent materials in the well region as a means of enhancing the local heating effect.
Yuan, in U.S. Pat. No. 6,631,761, suggests the use of electrode configurations around the well as a means of further controlling the heating effect in conjunction with RF probes, such as those suggested by Bridges, et al.
Both Haagensen, in U.S. Pat. No. 4,620,593, and Jeambey, in U.S. Pat. No. 4,912,971, propose true underground antennas for RF (and microwave) heating. Haagensen further proposes a modified waveguide to be placed within the well casing. The waveguide, however, at the only available, relevant microwave frequency is still far too large to fit within any standard well casing.
U.S. Pat. No. 5,109,927 to Supernaw describes the use of a hypothetical directional antenna to direct energy selectively at the bottom region of a production zone to improve steam recovery.
In general, RF thermal stimulation techniques have encountered several pitfalls. These pitfalls include localized charring around the heating probes, limited field penetration, electrical downhole component failure, and the like. These pitfalls have led to improvements in electrical components as well as attempts to create a more uniform energy distribution throughout the heating zone.
The use of acoustic energy to stimulate heavy oil recovery has been known for a considerably long time. U.S. Pat. No. 3,378,075 to Bodine and U.S. Pat. No. 4,437,518 to Williams describe the use of sonic transmitters as a means of stimulating oil well production. U.S. Pat. No. 2,670,801 to Sherborne is one of the earliest disclosures of the use of sonic energy for this purpose. Wesley, in U.S. Pat. No. 4,345,650, further discloses the use of an explosive, ablative, electric spark as a means of generating a high-intensity acoustic wave at or near a subsurface oil formation to stimulate oil production.
More recently, U.S. Pat. Nos. 6,186,228 and 6,279,653 to Wegener, et al., disclose the use of electro-acoustic transmitters inside a wellbore to improve oil production from an oil-bearing formation. U.S. Pat. Nos. 6,227,293 and 6,427,774 to Huffman, et al., and Thomas, et al., respectively, describe a means of generating coupled electromagnetic and acoustic pulses to stimulate oil production at much greater distances from the wellbore than was previously possible using direct acoustic generation within the wellbore. It is speculative if the electromagnetic pulse so generated could retain appreciable power density at the extended distances exceeding 6,000 feet. Meyer, et al., in U.S. Pat. No. 6,405,796, teaches the use of acoustic stimulation near the acoustic slow wave frequency in conjunction with fluid injection displacement as a means of stimulating oil flow. Abramov, et al., in U.S. Pat. No. 7,059,413, describe the use of a high intensity ultrasonic field near the bottom of the wellbore to generate heat and directly reduce the oil viscosity. This technique uses high frequency electrical heating of the well casing to maintain the oil at a relatively low viscosity.
Prior art techniques can have drawbacks.
The prior art techniques commonly use one or more stimulation techniques in conjunction with one or more wellbores drilled from the ground surface to intersect at least one oil-bearing stratum in a subterranean oil-bearing formation. The vertical string introduces several natural barriers which prevent the techniques from being commercially practical or at least introduces a large measure of additional cost or engineering difficulty related to energy loss and the necessity to locate the electrical equipment on the surface of the ground above the oil formation from where the energy must then be transmitted down a drill hole to access the oil formation. The barriers include inaccessibility of the stimulation device(s) after being placed, well completion at the surface and downhole end, operational unreliability of the stimulation device(s) and repair difficulties from location of the device(s) in the well casing, difficulty in keeping potentially harmful and/or flammable liquids from the device(s), well casing incompatibility with the stimulation actuators, creation of a means at the bottom of the drill casing whereby the energy can be transferred into the formation, and inability to recover the installed hardware. In particular, the limited size of standard drill casings, as well as the prohibitive cost of oversize casings, greatly restrict the size and complexity of components which can be reliably placed therein.
Prior art techniques seek to thermally stimulate the entire reservoir at one time followed by production from the entire reservoir over a period of up to five or ten years. To accomplish this, the entire reservoir must be thermally stimulated periodically over the production life of the reservoir. The unit of thermal energy required to produce a barrel of hydrocarbon-containing material can be relatively high. Moreover, heat can be lost heating up country rock and groundwater in proximity to the reservoir.
Many prior art techniques use vertical, rather than horizontal, hydrocarbon removal from the reservoir, along a typically long wellbore. Vertical hydrocarbon removal can raise recovery costs and lower recovery of hydrocarbons due to the pumping pressure and/or drive pressure (such as from steam introduced into the reservoir) required to overcome the effect of gravity.
Prior art techniques are generally unable to recover more than approximately 20% of the heavy oil in place, resulting in an overall inefficiency and loss of resource potential.